Thermal imaging or thermography is a recording process wherein images are generated by the use of imagewise modulated thermal energy.
A survey of "direct thermal" imaging methods is given e.g. in the book "Imaging Systems" by Kurt I. Jacobson-Ralph E. Jacobson, The Focal Press--London and New York (1976), Chapter VII under the heading "7.1 Thermography". Direct thermal thermography is concerned with materials which are substantially not photosensitive, but are sensitive to heat or thermosensitive. Imagewise applied heat is sufficient to bring about a visible change in a thermosensitive imaging material.
Most of the "direct" thermographic recording materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and a coloured image is produced.
According to U.S. Pat. No. 3,080,254 a typical heat-sensitive (thermographic) copy paper includes in the heat-sensitive layer a thermoplastic binder, e.g ethyl cellulose, a water-insoluble silver salt, e.g. silver stearate and an appropriate organic reducing agent, of which 4-methoxy-1-hydroxy-dihydronaphthalene is a representative. Localized heating of the sheet in the thermographic reproduction process, or for test purposes by momentary contact with a metal test bar heated to a suitable conversion temperature in the range of about 90-150.degree. C., causes a visible change to occur in the heat-sensitive layer. The initially white or lightly coloured layer is darkened to a brownish appearance at the heated area. In order to obtain a more neutral colour tone a heterocyclic organic toning agent such as phthalazinone is added to the composition of the heat-sensitive layer. Thermo-sensitive copying paper is used in "front-printing" or "back-printing" using infra-red radiation absorbed and transformed into heat in contacting infra-red light absorbing image areas of an original as illustrated in FIGS. 1 and 2 of U.S. Pat. No. 3,074,809.
DOS 2 400 224 and U.S. Pat. No. 3,708,378 each disclose in its example 1 the incorporation of benzotriazole in the thermosensitive element of a thermographic recording material comprising silver behenate, methyl gallate as co-reactant and tetrachlorophthalic acid anhydride.
WO 94/16361 discloses a multilayer heat-sensitive material for direct thermal recording for which no intermediate drying of organic noble metal salts is necessary and which is coatable from aqueous dispersions. This material comprises: a colour-forming amount of finely divided, solid colourless noble metal or iron salt of an organic acid distributed in a carrier composition; a color-developing amount of a cyclic or aromatic organic reducing agent, which at thermal copy and printing temperatures is capable of a colour-forming reaction with said noble metal or iron salt; and an image toning agent; characterized in that (a) the carrier composition comprises a substantially water-soluble polymeric carrier and a dispersing agent for the noble metal or iron salt and (b) the material comprises a protective overcoating layer for the colour-forming layer. This patent application also discloses that suitable antifoggants for use in the invention are well-known photographic antifoggants such as 2-mercaptobenzo-triazole, chromate, oxalate, carbonate, benzotriazole (BZT), 5-methylbenzo-triazole, 5,6-dimethylbenzotriazole, 5-bromobenzotriazole, 5-chlorobenzo-triazole, 5-nitrobenzotriazole, 4-nitro-6-chlorobenzotriazole, 5-nitro-6-chlorobenzotriazole, 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene, benzimidazole, 2-methylbenzimidazole, 5-nitrobenzimidazole, 1-phenyl-5-mercaptotetrazole (PMT), 2-mercaptobenzimidazole, 2-mercaptobenzothiazole, 2-mercaptobenzoxazole, 2-mercaptothiazoline, 2-mercapto-4-methyl-6,6'-dimethylpyrimidine, 1-ethyl-2-mercapto-5-amino-1,3,4-triazole, 1-ethyl-5-mercapto-1,2,3,4-tetrazole, 2,5-dimercapto-1,3,4-thiodiazole, 2-mercapto-5-amino-1,3,4-thiodiazole, dimethyldithiocarbamate, and diethyldithiocarbamate. Only mercaptobenzotriazole is used as an antifoggant in the examples of WO 94/16361.
Black monochrome images obtained with state of the art direct thermal recording materials, utilizing oxidation-reduction image-forming processes based on substantially light-insensitive organic silver salts and reducing agents therefor and having a protective layer to avoid image damage during image-wise application of heat to the material, in general exhibit a brownish tone and a non-neutral background when viewed using a viewbox, both of which becoming more pronounced upon storage making such materials unacceptable for medical applications. Furthermore, medical applications require prints with long term stability for legal reasons. The colour neutrality of black monochrome images can be quantified by spectrophotometric measurements according to ASTM Norm E179-90 in a R(45/0) geometry with evaluation according to ASTM Norm E308-90 to produce the CIELAB a* and b* coordinates and the colour neutrality of the maximum density can be quantified using the numerical colour value (NCV). The NCV value is defined as: ##EQU1## where D.sub.1, D.sub.2 and D.sub.3 are lowest, next highest and highest respectively of the optical densities measured with a MacBeth.TM. TR924 densitometer through blue, green and red filters. The larger the NCV value the better the colour neutrality, with maximal colour neutrality corresponding to a NCV-value of 1.
There is a need for direct thermal recording materials capable of producing images with a more neutral image colour and a more neutral image background which are maintained during storage and periodic viewing in transmission with a viewbox.